1. Field of the Invention
The present invention relates generally to the field of display devices. In particular, the present invention relates to an optical panel having a plurality of stacked planar optical waveguides for guiding light from an inlet face to an outlet face of the optical panel. More specifically, the present invention relates to a plurality of stacked planar optical waveguides for an optical panel, the planar optical waveguides comprising a core material having a gradient refractive index.
2. Description of the Background
Optical screens typically use cathode ray tubes (CRTs) for projecting images onto the screen. The standard screen has a width to height ratio of 4:3 with 525 vertical lines of resolution. An electron beam is scanned both horizontally and vertically across the screen to form a number of pixels which collectively form the image.
Conventional cathode ray tubes have a practical limit in size, and are relatively deep to accommodate the required electron gun. Larger screens are available which typically include various forms of image projection. However, such screens have various viewing shortcomings including limited viewing angle, resolution, brightness, and contrast, and such screens are typically relatively cumbersome in weight and shape. Furthermore, it is desirable for screens of any size to appear black in order to improve viewing contrast. However, it is impossible for direct view CRTs to actually be black because they utilize phosphors to form images, and those phosphors are non-black.
Optical panels may be made by stacking planar optical waveguides, each waveguide having a first end and a second end, wherein an outlet face is defined by the plurality of first ends, and wherein an inlet face is defined by the plurality of second ends. Such a panel may be thin in its depth compared to its height and width, and the cladding of the waveguides may be made black to increase the black surface area. As shown in FIG. 4, these optical panels typically comprise planar optical waveguides 10a of the type which include discrete cladding layers 82 directly adjacent to and surrounding core layers 80. The cladding layers 82 have an index of refraction which is discretely lower than that of the core layers 80 and thus enables transmission of light 22 by internal reflection. This results in discrete reflections, or bounces, of the light 22 at interfaces 95 between the cladding layers 82 and core layers 80. This optical waveguide configuration is of the type which will be referred to hereinafter as "step index cladding".
However, optical waveguides of the step index cladding type have at least two significant drawbacks. First, a small loss of light takes place at each bounce at the interface 95 between the core layer 80 and surrounding cladding layers 82. Although the loss of light at each bounce within the optical waveguide is extremely small, a light ray may undergo a large number of bounces as it traverses the core layer. Optimally, it is desired to have the core layer thickness to be as small as possible to achieve higher resolutions. But, as the core layer thickness decreases, the number of bounces the light ray must endure increases. Therefore, the amount of light loss that occurs in optical panels (and in particular, higher resolution optical panels), becomes a significant detriment to the overall efficiency and performance of the optical panel, as well as the quality (e.g. brightness, sharpness, etc. . . ) of the image.
FIG. 5 illustrates the second significant drawback of using optical waveguides of step index cladding type. When light 22 entering core layer 80 comprises two or more different wavelengths, a phenomenon known as chromatic dispersion results. As shown in the figure, light 22 comprising two different wavelengths, even entering the core layer 80 at the same angle, will be displaced when exiting the core layer 80 resulting in two corresponding light rays 22a, 22b. These light rays 22a, 22b exit the outlet face of the optical panel at slightly different exit angles resulting in poor color quality of the image. This means that the exit angle of the light at the outlet face of the optical panel is dependent on the wavelength, or color, of the components of the input light. As can be envisioned, this phenomenon is further exaggerated when the light path that a light ray follows through the core layer 80 increases. For example, the chromatic dispersion effect increases as the core layer 80 becomes longer (i.e. in the direction that the overall light travels therethrough) for larger optical panels. Thus, the chromatic dispersion that occurs in optical panels using optical waveguides of step index cladding type is another significant detriment to the performance of the optical panel, as well as the quality (e.g. color, sharpness, etc. . . ) of the image.
Therefore, the need exists for an optical panel which possesses the advantages corresponding to the use of stacked optical waveguides, but which does not suffer from the decrease in efficiency, performance and quality resulting from the light loss from the discreet bounces that the light undergoes in the optical waveguides of step index cladding type, nor suffer from the deleterious effects of chromatic dispersion when using optical waveguides of step index cladding type.